German Published Pat. (Offenlegungsschrift) No. 2,545,720 relates to highly crystalline thermoplastic block copolyesters which crystallize rapidly and have an intrinsic viscosity of at least 0.4 dl/g and which consist of
A. 60-95% by weight, relative to the sum of A and B, of copolyester segments based on terephthalic acid radicals, ethylene glycol radicals and co-diol radicals and PA1 B. 40-5% by weight, relative to the sum of A and B, of block segments, linked to A, with a glass transition temperature below 0.degree. C. and an average molecular weight of 400 to 1,000,000, A and B being linked to one another via carboxylate groups, characterized in that A consists of PA1 A. about 60 to 95 wt. % based on the combined weight of A and B of a homopolyethylene terephthalate with a minimum intrinsic viscosity of 0.2 dl/g (in 1:1 phenol/tetrachloroethane at 25.degree. C. and 0.5% strength), PA1 B. about 40 to 5 wt. % based on the combined weight of A and B of a polymer having: PA1 C. about 10 to 0.05 mol %, based on the mols of ethylene glycol residues in A, of a codiol having: PA1 n is a polymerization degree sufficiently high to give the defined ethylene glycol based polyester segment an intrinsic viscosity of at least about 0.2 dl/g (in 1:1 phenol/tetrachloroethane at 25.degree. C. and 0.5% strength), PA1 P is the reactive end group free residue of a polymer having a glass transition temperature below about 0.degree. C., reactive carboxyl, ester or hydroxyl groups and an average molecular weight of about 400 to 1,000,000, and PA1 D is the hydroxyl free residue of a codiol having 4 to 10 carbon atoms with the hydroxyl groups separated by 3 or 4 carbon atom branched or unbranched aliphatic residues and at least one of the hydroxyl groups secondary or tertiary or the diol carrying two primary hydroxyl units is unsubstituted, monoalkyl substituted or dialkyl substituted and the substituents, if any, contain a total of at least 4 carbon atoms.
(a) at least 90 mol %, relative to the acid component, of terephthalic acid radicals, PA2 (b) 90-99.5 mol %, relative to the sum (b)+(c), of ethylene glycol radicals and PA2 (c) 10-0.5 mol %, relative to the sum (b)+(c), of radicals of co-diols with 4-10 C atoms, the OH groups of which are separated by aliphatic branched or unbranched alkylene groups with 3 or 4 C atoms and which either PA2 1. carry at least one secondary or terteriary OH group or PA2 2. carry two primary OH groups and are unsubstituted, monoalkyl-substituted or dialkyl-substituted, the sum of the C atoms of the substituents in the case of substitution being at least 4. PA2 1. a glass transition temperature less than about 0.degree. C., PA2 2. reactive carboxyl, ester or hydroxyl end groups, and PA2 3. an average molecular weight of about 400 to 1,000,000, and PA2 1. 4 to 10 carbon atoms, PA2 2. 2 hydroxyl groups separated by aliphatic branched or unbranched radicals of 3 or 4 carbon atoms wherein
These copolyesters can be prepared in a manner which is in itself known by esterifying or transesterifying the dicarboxylic acids, preferably pure terephthalic acid, and/or the corresponding dimethyl esters with 1.05 to 4.0, preferably 1.8-3.6, mols of the diols, relative to 1 mol of the dicarboxylic acid component, with the addition of the polymer which is to be incorporated chemically as the block segment, in the presence of esterification and/or transesterification catalysts at between 150.degree. and 250.degree. C. (reaction step I) and subjecting the reaction products thus obtained to a polycondensation reaction under customary conditions, that is to say in the presence of esterification catalysts at between 200.degree. and 300.degree. C. under reduced pressure (&lt;1 mm Hg) (reaction step II).
A particularly preferred embodiment consists in admixing the co-diols, together with the block segment polymers, to the reaction mixture as late as possible, that is to say only after the reaction of terephthalic acid, or of its ester-forming derivatives, with ethylene glycol to give bis-(2-hydroxyethyl) terephthalate has taken place or, even more advantageously, only after a polyethylene terephthalate prepolymer which has a degree of polycondensation of more than 4 has formed. Thereafter, the mixture can then be subjected to a polycondensation reaction in the customary manner, as described above.